The ascent of the dinosaurs:
ecological release after the Triassic-Jurassic mass extinction in continental
environments

Paul E. Olsen and Emma C.
Rainforth

New data from early Mesozoic rift strata correlated by
Milankovitch cyclostratigraphy and magnetostratigraphy allow high-resolution
analysis of the Triassic-Jurassic (TJ) continental transition. Tetrapods
are represented by an extraordinary ichnological record (>10E^4 specimens)
and many new skeletal discoveries.

The Late Triassic saw an increase in tetrapod diversity,
but also high turnover rates with as many dinosaurian as non-dinosaurian
extinctions. Theropod (?ceratosaurian) dinosaurs increased in relative
abundance and maximum size through this time. End-Carnian tetrapod extinctions
(correlated to the marine magnetostratigraphically) are insignificant,
although there was an accelerated turnover within the Carnian. Large scale
zonal faunal and floral provinciality continued through the Triassic. The
Rhaetian relative overabundance of theropod compared to herbivore tracks
suggests ceratosaurians(?) were opportunistic carnivores, with diets dominated
by aquatic or semiaquatic prey.

Abundant and diverse ichnofaunules containing typical
Triassic forms occur ~10 ky below the palynologically-identified TJ boundary.
A >50% extinction occurred at the boundary, which also features an Ir anomaly
and fern spike. A similar pattern is known at coarser levels for osseous
taxa.

Earliest Jurassic (< 20 ky post-boundary) ichnological
assemblages have extraordinarily low diversity (3 ichnogenera in ~10E^3
specimens) with no herbivores; theropod track abundance soars together
with a 20% increasein maximum size marked by the appearance of Eubrontes
giganteus. In eastern North America, three new ichnogenera (including
two dinosaurian) appear within 400 ky, but there is virtually no change
in the succeeding 2 my. Herbivore footprints are numerically subordinate
to theropods, although less so with time and increasing latitude. Globally,
Early Jurassic osseous and ichnological faunas are nearly homogeneous,
even at the generic level.

We propose that the remarkable and relatively abrupt change
in maximum size of theropod tracks was a consequence of ecological release
upon extinction of competitors at the TJ boundary, and that the earliest
Jurassic theropod communities were part of an aquatic-based, post-catastrophe
ecosystem. Diversity did not recover to pre-boundary levels for more than
10 my.